Adapter plug



FIG. 1 is a front top and left-side perspective view of an adapter plug showing my new design;

FIG. 2 is a bottom plan view thereof;

FIG. 3 is a right-side elevational view thereof;

FIG. 4 is a rear elevational view thereof; and

FIG. 5 is a front elevational view thereof. 

The ornamental design for an adapter plug, as shown and described. 